1. Field of the Invention
This invention relates in general to heat exchange systems and, in particular, to an instantaneous steam fired water heater system.
2. Description of the Prior Art
Water heaters are used in a large variety of residential, commercial and industrial settings. For example, large storage tank systems are often found in hospitals and institutions, industrial plants, schools, universities, apartment complexes, and the like. The different types of water heaters which are used to supply hot water to these systems may use various sources of energy, such as gas, oil, electricity or steam. For example, in conventional gas/oil fired water heaters, hot gas flows through a series of vertically mounted tubes which are mounted in vertical fashion between top and bottom support plates within the water heater tank. Water flows into and out of a chamber located between the support plates and contacts and circulates about the exterior of the vertical tubes to effect heat transfer to heat the water.
U.S. Pat. No. 4,465,024, issued Aug. 14, 1984, and assigned to the assignee of the present invention describes another type of water heater which has a submerged, pressurized combustion chamber so that all combustion occurs in the water heater tank interior in a chamber surrounded by water, thereby reducing heat loss and increasing efficiency. These two examples are merely intended to be illustrative of the various types of gas/oil fired water heaters which exist in the prior art.
In some of the end applications mentioned above, such as, for example, in a municipal apartment complex or an industrial process, it may be more convenient and economical to utilize an existing source of steam or other hot fluid as the energy source for heating the water, rather than using gas or oil. There are a large number of prior art patents on devices in which liquids have been heated by fluids such as steam or other hot liquids. One of the common apparatus designs is the so called “shell-and-tube” heat exchanger. In conventional shell-and-tube heat exchangers, the tube section of the heat exchanger consists of a bundle of tubes which are open at both ends. At each end, the tubes extend through and are welded to a tube sheet. The shell of the heat exchanger completely encloses the bundle. The tubes within the bundle are spaced apart from each other, and from the shell, to define the shell-side section of the heat exchanger. In a typical heat exchanging operation, one of the fluids (liquid or gas) is passed through the tube section of the heat exchanger. The other fluid is then passed through the shell section, that is, on the outside of the tubes, often in a flow path which is countercurrent to the fluid flowing through the tube section.
Because the demand for hot water tends to vary over time in most installations, it is generally necessary to provide some sort of control over the heat source or flow rate through the device to accommodate the varying flow rates of the water being heated. In most applications the temperature of the water to be heated varies dramatically according to the time of year, and other factors. For example, a typical domestic hot water system is only under demand about 10-20% of the time. Additionally, in most applications the quantity of water flowing through the heat exchanger varies according to the time of day and use patterns of the application. Thus, the heat exchanger outlet water temperature must be regulated in order to accommodate variability resulting from the inlet water temperature and flow rate changes.
In the case of hot water storage systems utilizing bulk storage tanks, one type of existing control system is commonly referred to as the “feedback-type” system. These systems operate by sensing the temperature of the water in the storage tank, using a temperature sensing device in the tank to feed back instructions to a steam control valve. Because they respond to what has already happened, feedback systems are by their very nature reactive, i.e., they are a step behind demand. Thus, although they can supply a large volume of water, bulky tank systems are slow to react and waste energy to heat unused water. They also tend to require a large amount of floor space to accommodate the storage tank. Maintaining a leaking tank is expensive and replacement often involves tearing out walls of the surrounding structure.
A shell-and-tube heat exchanger of the appropriate design in an instantaneous feedback system can eliminate the need for a bulky hot water storage tank which can be subject to leaking and corrosion and can also eliminate the response time problem discussed above. In fact, tankless instantaneous feedback systems, i.e., a shell-and-tube heat exchanger with a temperature regulator, are known in the prior art. However, the typical tankless instantaneous feed back system of the prior art have generally attempted to control outlet water temperature by controlling steam flow through the heat exchanger. Because of the inherent thermal lag in this type control process, outlet water temperatures can vary greatly when faced with wide swings in inlet water temperature and flow. These swings can exceed 50% of the difference between the cold water temperature and the desired hot water temperature when flow through the water heater changes quickly from full water flow capability to zero flow and back to full flow.
Thus, despite the improvement in the art generally in the area of heat exchangers and water heaters of the above type, a need continues to exist for an improved instantaneous water heater design which utilizes steam from a local boiler or district plant to provide energy to heat water to a desired temperature for a variety of hot water end use applications and which solves the problems of thermal lag discussed above.
A need also exists for such a heat exchange system which provides redundancy in the case of failure of a major component of the system.
A need also exists for such a heat exchange system which provides adequate safety features to prevent any danger of scalding should a valve or other component of the system fail.